1. Field of the Invention
The present invention relates to an image processing apparatus, an electronic signature generation system, an electronic signature key generation method, an image processing method, and a program.
2. Description of the Related Art
In general, in order to verify the authorship of a document, the author places a signature, a seal, or the like (hereinafter referred to as a signature) on that document. This signature is an indication that the person who signed takes responsibility for the contents described in that document. If the document is a paper document, the author of the document adds their handwritten signature. On the other hand, if the document is an electronic document, it is difficult for the signer to impress a seal on electronic data or place his/her autograph thereon. For this reason, a method of attaching data called an electronic signature to uniquely identify signers by these electronic signatures is used. In recent years, various documents are digitalized and the importance of electronic signatures is increasing. In such a circumstance, there are many cases where resistance to the forgery of electronic signatures becomes a problem. Although signatures attached to paper documents have the same problem, more caution is necessary for electronic signatures since electronic data can be reproduced more easily.
A well known example of an electronic signature scheme is the ElGamal signature scheme. This scheme is based on the difficulty of the discrete logarithm problem. First, a signer generates a signing key for generating an electronic signature and a verification key for verifying the validity of the electronic signature. Furthermore, the signer makes the verification key available to the public. Then, the signer generates the electronic signature using the signing key and an electronic document and provides the electronic signature to a verifier together with the electronic document. For this reason, the verifier is able to verify the electronic signature using the verification key available to the public. In the case of the ElGamal signature scheme, when one tries to generate a signing key or an electronic signature from a verification key, it is necessary to solve a discrete logarithm problem which is difficult to solve because it involves a large amount of computation. The same difficulty lies in generating a signing key from an electronic signature. Such a difficulty prevents the forgery of signatures.
However, if the signing key is revealed to a third party for some reason, the third party is able to forge the electronic signature freely. Therefore, when the signing key is revealed, it is difficult to differentiate whether the electronic signature generated with the signing key belongs to the true signer or it is forged by the third party. In the case of signatures attached to paper documents, it is possible to differentiate reproduction from the original relatively easily. However, in the case of electronic data, it is difficult to differentiate them perfectly. For this reason, a countermeasure is adopted to invalidate the electronic signature, the signing key, and the verification key at any stage where the revelation of the signing key is discovered. At that time, electronic documents having the electronic signature attached thereto are also made substantially invalid.
Various schemes have been proposed to reduce problems associated with the revelation of the signing key. For example, Japanese Patent No. 3640785 describes a method that sets different signing keys for each period so that the respective signing keys are valid for only electronic signatures generated during each period (see FIG. 21). According to this method, when a signing key is revealed and thus that signing key is disused, signing keys for periods earlier or later than the period corresponding to that signing key can be maintained as valid as they were. For this reason, it is not necessary to invalidate electronic signatures generated during other periods other than the period corresponding to the revealed signing key and electronic documents to which that electronic signature is attached. As a result, it is possible to reduce the number of electronic documents invalidated.